1. Field of the Invention
The present invention relates to a tire balance measuring device for measuring a balance of a tire.
2. Description of the Related Art
As devices for measuring a balance of a tire, there has been known a tire balance measuring device having a structure to rotate a tire inserted between two rims that are arranged one above the other. In the thus-structured tire balance measuring device, an upper rim should be completely separated from a lower rim when the tire is attached or detached, to prevent the upper rim from becoming detached due to the pressure of air supplied to the tire at the time of measurement of the balance. To achieve this in the tire balance measuring device, a lock shaft to which the upper rim is attached is inserted to a predetermined level into a fitting hole formed in a spindle to which the lower rim is attached, and fixed to the spindle through engagement therewith (refer to, for example, Japan Patent No. 3904319).
In the devices as described above, a predetermined fitting allowance is provided between the lock shaft and the fitting hole in the spindle to facilitate insertion and withdrawal of the lock shaft into/from the spindle. When the tire is inflated by supplying air into the tire at the time of measurement of the balance, the lock shaft is inclined relative to the fitting hole in the spindle due to the presence of a gap corresponding to the fitting allowance, with the result that the upper rim is accordingly inclined relative to the lower rim. The above-described devices suffer from a problem in that measured values of the balance have an error introduced by an occurrence of the inclination. In addition, because the lock shaft can be inclined in a completely random direction every time the tire is supplied with air, it is difficult to correct for variations in the measured values depending on a direction of the inclination.
To address the problem, Japan Patent No. 3133465 discloses a tire balance measuring device in which three or more contact portions are protrudingly formed on an outer circumferential surface of a fitted shaft (the lock shaft). This Japan Patent further describes that the above-described structure can function to limit the direction in which the fitted shaft is inclined relative to the fitting hole, which makes it possible to make a correction to the measured value.
However, even with the structure described in Japan Patent No. 3133465, the upper rim can not be limited sufficiently for preventing irregular inclination that occurs due to the presence of the fitting allowance between the fitted shaft and the fitting hole. It is assumed, for example, that the tire balance measuring device of the above-described structure includes three contact portions on the outer circumferential surface of the fitted shaft. In this case, the direction in which the fitted shaft can be inclined is toward any midpoint position between the adjacent contact portions, which means that there are three possible directions of inclination of the fitted shaft in total. In which of the three directions the fitted shaft will be inclined when the tire is supplied with air remains unknown until air is actually introduced into the tire.
The present invention, which was made in view of the aforesaid present circumstances, therefore aims to provide a tire balance measuring device in which even when the fitted shaft (a lock shaft) could be inclined in a random direction at an angle created by the gap corresponding to the fitting allowance every time the tire is supplied with air, an influence exerted due to the inclination on the measured value can be minimized.